1. Field of the Invention
The invention relates generally to electrical power systems, and more particularly to systems and methods for reducing harmonic distortion in a power system resulting from non-linear loading on the power system.
2. Related Art
Variable speed drives are commonly used in the petrochemical industry to power electric submersible pumps. These pumps are often used in downhole applications such as pumping oil from wells. Because variable speed drives provide advantages such as improved energy usage, improved process control, increased production and increased reliability with respect to competing technologies, the use of variable speed drives has increased substantially in recent years.
All the variable speed drives have a number of advantages, there are also some disadvantages associated with them. Because a variable speed drive is a non-linear load, the drive draws current from its power source (e.g., a utility power grid) in a non-linear fashion. This, in turn, causes the power source to experience distortion as it provides the non-linear current to the variable speed drive. This distortion can cause the supply voltage to be distorted (non-sinusoidal) which can then result in problems such as overloaded electrical distribution equipment (e.g., transformers) tripped circuit breakers, resonance with power factor correction capacitors, interference, and so on. These problems are not simply local to the variable speed drive, but also affect the other consumers that receive power from the power source.
In order to minimize the negative effects of harmonic distortion which are caused by non-linear loads such as variable speed drives, a standard (IEEE 519) was developed and established to define limits for the amount of distortion that can be caused in a power system. This standard lead to the implementation of controls in some equipment to reduce the distortion caused by non-linear loading in the equipment. These controls may, for example, implement harmonic filtering using passive filters.
Passive harmonic filters may work very well in a number of applications, but they also have a number of disadvantages. For instance, passive harmonic filters work well if the harmonics of interest are well known and do not vary with time, but variable speed drives which are used in applications such as electric submersible pumps change operating frequencies and consequently produce changing harmonics. Further, if power is interrupted, passive filters normally experience in a large and potentially damaging inrush of current when power is restored. Also, passive filters may themselves generate harmonics due to resonance in their components. Still further, passive filters normally attenuate not only the unwanted harmonics, but also the fundamental frequency of the supplied power.
It would therefore be desirable to provide systems in methods for reducing harmonic distortion that arises in power systems by non-linear loads which do not suffer from the disadvantages of passive harmonic filters. It would also be desirable to provide systems and methods for reducing harmonic distortion which can be implemented in conjunction with existing equipment which does not already incorporate mechanisms for reducing or eliminating harmonic distortion.